Welcome to the forum, niceaction!

Advance Purchase fares are (as of today) only available up until the 10th November. If you can provide more details of the dates/times you wish to travel, both out and return, I can look at the options for a flexible walk-up fare.
Advance Purchase tickets limit you travel on the booked train(s) only and are normally available up to 12 weeks before. If you require flexibilty with your travel plans, then a walk up ticket (Anytime Return, Off Peak Return, Super Off Peak Return - all permit return within one month) wil be a better option, but will cost you more. If cost is your main consideration then you'll need to wait until Advance Purchase booking opens for your dates of travel. There may also be options for 'split ticketing' or a different route.